The production of alkylene oxides from alkenes in the presence of suitable catalysts is well known. Brian J. Ozero, Handbook of Chemicals Production Processes, edited by Robert Meyers, McGraw Hill Book Co. (1986) at Chapter 1.5, discloses cyclic processes using both oxygen and air as an oxidant for the production ethylene oxide from ethylene. In these processes, the alkene is oxidized in a multitubular catalytic reactor in vapor phase. The reactor off gases are cooled and scrubbed with water in an absorber to recover ethylene oxide which is sent to a recovery section for further purification.
In the oxygen-based process described by Ozero, the scrubber off gases are divided into three parts which are respectively: i) recycled to the reactor, ii) vented and iii) sent to a separator for carbon dioxide removal and recycle of the remaining hydrocarbons. This process suffers from several disadvantages. In particular, the process requires a separate carbon dioxide removal unit and a purge to remove argon which would otherwise accumulate in the system.
In the air-based process described by Ozero, the scrubber off gases are sent to a second reactor, which is the purge reactor, where additional unreacted ethylene is reacted using a higher air to ethylene ratio, thereby foregoing some ethylene oxide selectivity. The reactor off gases are passed through another water scrubber to recover ethylene oxide.
It is known that the volume of hydrocarbons purged, when utilizing air as a source of oxygen, requires that the purge scrubber off gases be incinerated to remove any remaining hydrocarbons in order to meet environmental regulations. In this air-based process, an additional purge oxidation reactor, a water scrubber, and an effluent incinerator are required, as well as a greater volume of catalyst. Another shortcoming of the processes described by Ozero is that they are for practical purposes limited to the use of either oxygen or air. It would be advantageous to eliminate the purge and additional carbon dioxide separator and operate the ethylene oxide reactor at a higher selectivity to improve the overall process efficiency.